Type 1 diabetes mellitus (TIDM) is an autoimmune disease. In the past, several trials of immunosuppressive treatments have shown some efficacy in improving the natural history of disease but have not shown long term benefits. Nonetheless, animal studies strongly argue that immune intervention can prevent and even reverse the disease. This is a phase 1 trial of a new immunosuppressive agent (hOKT3yl(Ala-Ala)) in patients with TIDM. This drug differs in its mode of action from previous tested agents. In animal studies of the human disease, a similar agent has reversed diabetes after its clinical onset. Unlike other immune suppressive agents, this new drug does not have the side effects, which have precluded their application to this clinical setting. In studies with this agent for treatment of kidney rejection, the drug has been shown to be effective and without the toxicity of drugs such as Cyclosporine A, FK-506, or OKT3. The purpose of this phase I study is to determine the doseage and test the safety, tolerability, and immune effects of hOKT3yl(Ala-Ala) in patients with TIDM. In addition, we will extend our studies to test the efficacy of the drug in preventing the loss of insulin secretory capacity that occurs during the first 2 years of TIDM. The study subjects include patients between the ages of 13 and 45 within the first 6 weeks following diagnosis of TIDM. They will receive the drug over a 14 day period at the General Clinical Research Center. Metabolic, immunologic, and other studies will be conducted over a 2 year follow-up period. A control group will not receive the drug but will be asked to participate in metabolic testing.